A variety of vehicles, whether airborne or land-based, include a primary power plant or engine which is operated under a variable speed regime and capable of generating upwards of 1500 h.p. Many of the vehicles also have a much smaller secondary power plant or engine which is operated to generate electrical power to operate systems when the primary engine is shut down (i.e. in order to save fuel, "run silently", or prevent detection). Such a secondary engine may only produce 100 h.p.
Many of the electrical systems used in such vehicles require constant frequency electrical power. In order to provide such electrical power, generators have been included with such primary engines in the form of either a constant speed drive (CSD) or a variable speed constant frequency (VSCF) converter. The constant speed drive regulates the speed at which a primary engine-driven AC generator is rotated and thus delivers constant frequency. The variable speed constant frequency converter regulates the frequency that a generator delivers while being rotated at variable speeds by the primary engine. These speed or frequency conversion devices are relatively expensive and reduce the overall efficiency and reliability of the generating system.
In order to eliminate the above devices and desensitize the system from the effects of frequency, other vehicle systems implement high voltage DC power for operation. Such devices are more frequently found in state-of-the-art vehicles such as high technology aircraft and land-based vehicles such as tanks. Some of these vehicles further require that the generator used to generate the high voltage DC power (typically 270 Volts DC) should be capable of starting the primary engine as a back-up feature. In other words, once the primary engine is started, it runs the generator to satisfy the power requirements. Additionally, when necessary, the generator can be employed as a starter motor to restart the primary engine.
One way to satisfy the operating requirements for such applications is to provide a second 270 Volt DC generator, mounted on and driven by an auxiliary power unit (APU) which is carried on the vehicle for running the generator as a starter motor. A problem arises in that in order to motorize the salient-pole brushless DC generator from the available 270 Volt DC power, a complex inverter is needed, which defeats the purpose of adapting the 270 Volt DC power.
For example, a device as shown in U.S. Pat. No. 4,743,776 to Baehler et al. issued May 10, 1988 shows a device which attempts to overcome the aforementioned problems. However, the device in Baehler et al. is highly complex requiring a torque converter, two overrunning clutches, a field shorting switch and a torque converter pump. Such complexities create dramatic inefficiencies in the system, additional failure points, added costs, as well as substantial increases in added space and weight. It should be noted that the space and weight factors are extremely important in high technology vehicles such as aircraft and tanks since these factors have a critical effect on payload, mission range, and speed.
Two devices as shown in U.S. Pat. No. 4,743,777 to Shilling et al. issued May 10, 1988 and U.S. Pat. No. 5,055,700 to Dhyanchand issued Oct. 8, 1991 include the complexities as discussed above with reference to Baehler et al. as well as variable voltage, variable frequency inverters. More specifically, Shilling et al. is highly complex including dual exciter windings, a rotor position sensor, complex microprocessor logic devices, as well as the converter/inverter mentioned hereinabove. The device as shown in Dhyanchand uses a variable voltage, variable frequency inverter which results in a reduction in the efficiency of the operation of the device. Further, the field shorting switch as shown in Dhyanchand requires a deliberate action in order to actuate the switch. Additionally, Dhyanchand uses an additional squirrel-cage circuit which clearly requires additional elements, space, and weight.
The problem of electrically starting the primary engine by using the engine-mounted main generator as a starter motor has been addressed by various means with varying levels of success as discussed hereinabove. When dealing with available DC power and a brushless DC generator, one solution is to motorize the device like a brushless DC motor. This starting method, however, has not been considered to be the best solution because it was thought to require extensive modifications in the engine-mounted generator. Also, such a system was thought to require use of inverters and other power conversion devices. Use of these additional devices would appear to defeat the purpose of using a 270 Volt DC power system because one of the advantages of implementing such a power system is to eliminate inverters and other power conversion devices.